Re: [Ifeffit] question about Eo and simultaneous fitting
Hi again to everybody,
Thank you for the answers. They are all helpful.
Just to specify a little more: all samples are measured at the K edge of the
doped metal oxide. I expect relation between the coordination environments of
the different samples as the reduced and oxidized samples are mixture of two
formal oxidation states.
I will try with different Eo and the Shelly's aligning-suggestion.
Tony
- Original Message
From: Matt Newville <[EMAIL PROTECTED]>
To: XAFS Analysis using Ifeffit
Sent: Wednesday, February 14, 2007 9:01:29 PM
Subject: Re: [Ifeffit] question about Eo and simultaneous fitting
Tony --
> Should I fit EXAFS spectra of a doped material and the same material, but
> oxidized and reduced (three spectra) simultaneously? In my opinion: yes, as
> I am looking for small differences. Is varying one Eo for the three samples
> the right approach?
> I am also wondering, should I vary one Eo for samples with different doping
> concentrations?
>
I think you need to be more specific about what your samples are and
what you're trying to learn from them. For example, I could read
your question as meaning you have samples of
Mn K edge for Mn doped into silicon ("a doped material")
Si K edge of pure silicon ("same material, reduced")
Si K edge of quartz ("same material, oxidized")
In which case, I don't see that see much to be gained by fitting the
spectra together.
In general, one fits multiple spectra together when it is expected
that there is a relationship between the different coordination
environments, most usually a simple relationship between the g(R)'s
for the different samples.
E0 is typically different for each sample. There are situations where
they can be constrained to be the same, such as spectra are all on the
same sample at different temperatures.
For more specific answers, please ask more specific questions.
--Matt
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Re: [Ifeffit] question about Eo and simultaneous fitting
Tony --
> Should I fit EXAFS spectra of a doped material and the same material, but
> oxidized and reduced (three spectra) simultaneously? In my opinion: yes, as
> I am looking for small differences. Is varying one Eo for the three samples
> the right approach?
> I am also wondering, should I vary one Eo for samples with different doping
> concentrations?
>
I think you need to be more specific about what your samples are and
what you're trying to learn from them. For example, I could read
your question as meaning you have samples of
Mn K edge for Mn doped into silicon ("a doped material")
Si K edge of pure silicon ("same material, reduced")
Si K edge of quartz ("same material, oxidized")
In which case, I don't see that see much to be gained by fitting the
spectra together.
In general, one fits multiple spectra together when it is expected
that there is a relationship between the different coordination
environments, most usually a simple relationship between the g(R)'s
for the different samples.
E0 is typically different for each sample. There are situations where
they can be constrained to be the same, such as spectra are all on the
same sample at different temperatures.
For more specific answers, please ask more specific questions.
--Matt
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Ifeffit mailing list
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Re: [Ifeffit] question about Eo and simultaneous fitting
It is true that in many cases oxidation/reduction affects the low k-range behavior in chi(k), but if the second shell (metal-metal) frequency is not affected, the data can still be aligned by focusing at the high k-portion of the data, so what Shelly suggested, may be, in principle, done. Anatoly -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Scott Calvin Sent: Wednesday, February 14, 2007 10:55 AM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] question about Eo and simultaneous fitting Hi Shelly, Interesting--I've never tried that method with samples that I expect to differ in oxidation state. If it's differing in oxidation state, the local environment is probably different too...particularly, nearest-neighbor distances should be changed significantly. So how do you align the chi(k) data with each other, when they oscillate at a different frequency? --Scott Calvin Sarah Lawrence College At 10:38 AM 2/14/2007, you wrote: >The approach that I like to take, is to vary the choice of E0 in the >background subtraction step so that the chi(k) spectra are well aligned >with each other and the theory. Then only one Ezero parameter is >needed. > >Cheers, >Shelly ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] question about Eo and simultaneous fitting
Hi Scott, The change in frequency is a bigger affect at high k than the energy shift parameter. The energy shift parameter affects the data more strongly at low k. So I align the data to each other at low k, and then they become out of phase with each other at high k. A similar effect is seen for pressure dependent data where the oxidation state doesn't change but the bond length changes. I also look at how well the data are both aligned to the theory used to describe the data sets independently. The energy shift values should overlap within the uncertainty to justify using a single parameter for them. I suppose that my method might not work for all systems. But if the data are similar enough so that a multiple data set fit has significant overlap in the models used to describe the spectra, then it has worked. Shelly > -Original Message- > From: [EMAIL PROTECTED] [mailto:ifeffit- > [EMAIL PROTECTED] On Behalf Of Scott Calvin > Sent: Wednesday, February 14, 2007 9:55 AM > To: XAFS Analysis using Ifeffit > Subject: Re: [Ifeffit] question about Eo and simultaneous fitting > > Hi Shelly, > > Interesting--I've never tried that method with samples that I expect > to differ in oxidation state. If it's differing in oxidation state, > the local environment is probably different too...particularly, > nearest-neighbor distances should be changed significantly. So how do > you align the chi(k) data with each other, when they oscillate at a > different frequency? > > --Scott Calvin > Sarah Lawrence College > > At 10:38 AM 2/14/2007, you wrote: > > >The approach that I like to take, is to vary the choice of E0 in the > >background subtraction step so that the chi(k) spectra are well aligned > >with each other and the theory. Then only one Ezero parameter is > >needed. > > > >Cheers, > >Shelly > > ___ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] question about Eo and simultaneous fitting
Hi Shelly, Interesting--I've never tried that method with samples that I expect to differ in oxidation state. If it's differing in oxidation state, the local environment is probably different too...particularly, nearest-neighbor distances should be changed significantly. So how do you align the chi(k) data with each other, when they oscillate at a different frequency? --Scott Calvin Sarah Lawrence College At 10:38 AM 2/14/2007, you wrote: >The approach that I like to take, is to vary the choice of E0 in the >background subtraction step so that the chi(k) spectra are well aligned >with each other and the theory. Then only one Ezero parameter is >needed. > >Cheers, >Shelly ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] question about Eo and simultaneous fitting
Hi Tony, The approach that I like to take, is to vary the choice of E0 in the background subtraction step so that the chi(k) spectra are well aligned with each other and the theory. Then only one Ezero parameter is needed. Cheers, Shelly > -Original Message- > From: [EMAIL PROTECTED] [mailto:ifeffit- > [EMAIL PROTECTED] On Behalf Of Scott Calvin > Sent: Wednesday, February 14, 2007 7:52 AM > To: XAFS Analysis using Ifeffit > Subject: Re: [Ifeffit] question about Eo and simultaneous fitting > > Hi Tony, > > A quick answer before I try to fight my way through an ice storm to > get to work. :) > > Fitting the three spectra simultaneously makes sense, but if > oxidation state is expected to be different, then you really need > different E0's. > > As far as doping, it depends on the details. In some cases, it may > require a different E0's, in others, not. Of course, you can try it > both ways, and see what the fits think. > > --Scott Calvin > Sarah Lawrence College > > At 05:04 AM 2/14/2007, you wrote: > >Hi, > > > >first I would like to thank all experienced people answering in > >this forum!! I have learned really a lot from you. > > > >And now my question: > >Should I fit EXAFS spectra of a doped material and the same > >material, but oxidized and reduced (three spectra) simultaneously? > >In my opinion: yes, as I am looking for small differences. Is > >varying one Eo for the three samples the right approach? > >I am also wondering, should I vary one Eo for samples with different > >doping concentrations? > > > > > ___ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] question about Eo and simultaneous fitting
Hi Tony, A quick answer before I try to fight my way through an ice storm to get to work. :) Fitting the three spectra simultaneously makes sense, but if oxidation state is expected to be different, then you really need different E0's. As far as doping, it depends on the details. In some cases, it may require a different E0's, in others, not. Of course, you can try it both ways, and see what the fits think. --Scott Calvin Sarah Lawrence College At 05:04 AM 2/14/2007, you wrote: >Hi, > >first I would like to thank all experienced people answering in >this forum!! I have learned really a lot from you. > >And now my question: >Should I fit EXAFS spectra of a doped material and the same >material, but oxidized and reduced (three spectra) simultaneously? >In my opinion: yes, as I am looking for small differences. Is >varying one Eo for the three samples the right approach? >I am also wondering, should I vary one Eo for samples with different >doping concentrations? > ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
[Ifeffit] question about Eo and simultaneous fitting
Hi, first I would like to thank all experienced people answering in this forum!! I have learned really a lot from you. And now my question: Should I fit EXAFS spectra of a doped material and the same material, but oxidized and reduced (three spectra) simultaneously? In my opinion: yes, as I am looking for small differences. Is varying one Eo for the three samples the right approach? I am also wondering, should I vary one Eo for samples with different doping concentrations? Thank you for any hint! Tony It's here! Your new message! Get new email alerts with the free Yahoo! Toolbar. http://tools.search.yahoo.com/toolbar/features/mail/___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
